为什么前面有个and

learn more about the ... types...
learn more about which (of the types) is best...

Learn more about the different types of boilers.
Learn more about the different types of which would suit you best in our full guide to the various types of boilers.
拆成两句看好懂点。

就是动词learn的宾语而已。learn有两个并列宾语 ，（分别就是and 前和后两个宾语）。

你直接读成汉语 哪个

i dont know which tastes better.
我不知道哪个好吃

Learn more about which would suit you best。这句话是完整的，which引导名词性从句(可以整体看成一个名词)做about的宾语，指最适合的那一种，属于陈述句语序。which指(前文中提到的)从the different types of boiler中选的一种或者几种。